Marshall Valvestate 8100 pre-amp stage problem

[lyngaaskhan]

I could use some advice to troubleshoot this issue. I have a Marshall Valvestate 100v Model 8100. The pre-amp tube glows (heater circuit good), all led's light correctly based on switch positions, unit powers up. If I run a guitar through the input, the only way I get any sound to the speaker is with gain and volume cranked, and then it's only when I play hard that I get a very buzzy and low level signal. If I plug into the effects return, the power amp section works fine. I've done the basics as far as cleaning all of the jacks, particularly the effects send. I've tested the jacks with a meter and they seem to be operating correctly with a jack inserted and without. I've poked around and tested various connection and traces for cold solder joints, though I'm guessing that a simple continuity test may not indicate a good solder connection. Also, no obvious buned or shorted items anywhere. Board is very clean.

<edit>
I forgot to mention that I ran a patch cable from the effects send to the effects return to see if I got anything. I also ran the effects send into a line in on my mixer - again, nothing.
</edit>

Since I have signal post effects return jack, I've been working backword attempting to test components. All resistors seems to check out within tolerance. I can't seem to get a reading from some of the caps.

Any advice on which components might be likely suspects or a better method for troubleshooting?

Thanks,

Eric

[Enzo]

Two approaches. Either way, the preamp has the fault, while the power amp works. You have no signal at the FX send.

One approach is to apply a signal to the amp input, and trace it through the amp stage by stage, looking for the point it disappears. Alternatively, start at the FX send, injecting a signal, then move the injection point back stage by stage until you find the point the injected signal no longer passes.


In my world, when I have a dead preamp, before tracing, I just go down the row of op amps and look at the output pins, looking for any with unwanted DC offset. DC on an output usually means a failed IC. Of course, some ICs drive LEDs or something so they would have DC normally.

[lyngaaskhan]

Thanks, Enzo. There's a dual op-amp that is at the end of both clean and overdrive channels - each channel going through one side of he chip. I'll test it out. If it's bad, I'm concerned about why it's bad. There doesn't seem to be any other bad components around it. My experience with op amps is in effects pedals, and I haven't had any go bad (power regulators, on the other hand...). What sort of thing would cause failure, given that there are 9 or 10 op -amps in this head that are on the same power circuit?

[Enzo]

And IC1 is at the beginning of each channel. It is also common to both therefore.

IC5 is at the end of the two channels, and it is not just an op amp, it is an analog switch, a 5201.

That is why we trace the signal either by tracing(following) or by injection.

WHy would a chip fail? Most times you will never know. In a number of Marshall op amp designs, there is a mute contact on the input jack that goes right to an op amp in the mioddle of the circuit. A static hit on that input jack can damage an IC. But your mute line goes to the grid of the tube, and there is no damaging a tube with static. SO if the input IC is damaged, well, the most common failure is whatever device is closest to the real world. IC1 is exposed to whatever the input jack offers, while IC3 for example is out in the middle of stuff, not exposed.

Why might IC5 fail? beats me, I'd not worry about it, it happens so seldom.


That all the ICs are on the same power bus strikes me as irrelevant really. I don;t expect power supply trouble to kill them.

Unless I have reason to think otherwise, I just assume "random component failure" and leave it at that. If I had to predict any repeat failures in an amp like that, it would be the output transistors in the power amp. And if anything is exposed to the real world, it certainly is them.

[lyngaaskhan]

Excellent information and advice, as always. It's funny, I was just reading up on the 5201 at IC5. I now understand the input at pin 1 as coming from the channel switch. I'm going to work from the input toward the effects send. It seems to me that some issue with the input would be more likely the cause of input IC damage. I do not know anything about the history of this head. I've seen some 'interesting' methods employed to destroy amps over the years. A lot of running speaker out to line in. I'm sure you've seen it all in your shop, Enzo. Last week I repaired a Line 6 Spider II head for my teenage son's band. They burned up the output transistors by running the speaker out to the speaker out of a combo amp. Apparently, they thought that the jacks labelled "speaker" meant they could use the speaker in the combo. They also thought they needed to have both amps powered up. Frankly, I was surprised to hear it worked for nearly 30 minutes. I don't particularly care for that amp. Perhaps is the ad/da conversion, but it's very sterile sounding. Sound clips of the Marshall valvestate 8100 sound much better, so I have higher hopes for it. My son found it on Craigslist for cheap, so I agreed to take a chance on it and attempt a repair. There is really nothing worthwhile in his budget range other than a broken amp. He gets my time for free. I tell him he'll be taking care of me in my declining years as payment.

[Enzo]

Find him a used, beat up Peavey Bandit. World's most popular amp. They've made them for years. The more recent ones with the "transtube" circuitry sound pretty darn good.

[lyngaaskhan]

Okay, so I clearly need some proper troubleshooting gear. I made a poor mans signal tracer out of a broken 1/4 instrument cable and a .1 uf 25v capacitor I had laying around. I ran a small keyboard with a latched tone to the input jack use as my signal, plugged the 'tracer' into my mixer, and started tracing the signal from the input jack on. First thing I discover is the input jack is wonky. Cuts in and out with some wiggling. re-flowed the solder, cleaned with deoxit, still cuts out, so it gets replaced. Moving down the line, I discover that nearly all the pots are scratchy and several are cutting the signal. The amp is working now, but I'll be taking several passes at cleaning to see if the pots are salvageable, otherwise, I'll replace them all. Seems the main culprit was the dry/effects balance control in the send/return loop. I got fooled by getting signal from the return and messing with the mix. The pot is cutting the signal in some positions. Perhaps the sensible thing to do next time is clean all the pots and jacks as the first troubleshooting step. I learn every time.

[Enzo]

Very good, you did exactly what we might have suggested. I have a number of dead little cheap practice amps like come with cheap guitar "starter sets." Easy to fix, I make a signal tracer out of one when I need to. And your method is exactly it. If one googles "signal tracer" you get a zillion variations on it.

I always use a .047/630v cap for the job. The values don;t matter, but I have hundreds of those, so they are handy. A 25v cap is probably fine here, but don't use one when working in tube amps, use a higher voltage cap for that. And I usuall add something like a 1 meg or a 470k from the tracer end of the cap to ground to give the cap a ground reference for DC.


ANd another tip, if your power amp works, you could probably plug your tracer probe into the power amp in jack and use the amps own power amp to trace the preamp. SO you don't always need a second amp.


When I am just looking for a broken signal path, I don't care what it sounds like at the moment, all I care about is does the signal pass. SO while I have plenty of fancy test gear, many time my test signal is nothing more than me touching the input with my finger to inject HUM. Hum is as good as anything else as a test signal. And darn cheap.

And nothing wrong with a keyboard. Back when the whole world smoked, match books were everywhere, and if we needed a steady tone, we could jam a matchboook between keys on the organ, and voila, a steady tone. I have a cheap little Yamaha PSR something. Basic little keyboard with the half size keys. Used to lean against my bench leg and I indeed used it for test signals all the time. A roll of solder sitting on a black key was a steady tone if I needed one. Otherwise I sometimes triggered the demo song.


Most pots will clean up, and don;t forget to resolder them too.

[lyngaaskhan]

Input jack replaced, pots cleaned and reflowed. Working great!

Found something interesting when I had my son try it out. I had tested with one guitar and patch cord. My son pulled out
his guitar and I pulled the cord from the keyboard I'd been using as my signal source. When he plugged in, we got no very little distortion compared to the other guitar, regardless of settings. I swapped cords and everything was good. I'll resolder the ends if the cord and see what happens, but I find it odd that the cord seems to work in general, never noticed any problems with it, but killed the distortion on this amp with no noticeable gain drop.

[TheB3Bob]

I havr a similar problem, but one better.

1. I got this 8100 running, and passing audio (test tones). When I plug a guitar in the same jack, nothing. Dead silent. Any thoughts, ideas, help would be much appreciated.
   FX return (power amp in) passes audio. Problem in preamp. Im thinking follow Enzo throuvh the op amp troubleshoot process. Any suggestions??

[g1]

It gives sound with test tones at main input, but not with guitar plugged in to same input? What voltage level test tones were you using?

[vintagekiki]

I could use some advice to troubleshoot this issue. I have a Marshall Valvestate 100v Model 8100. The pre-amp tube glows (heater circuit good), all led's light correctly based on switch positions, unit powers up. If I run a guitar through the input, the only way I get any sound to the speaker is with gain and volume cranked, and then it's only when I play hard that I get a very buzzy and low level signal. If I plug into the effects return, the power amp section works fine. I've done the basics as far as cleaning all of the jacks, particularly the effects send. I've tested the jacks with a meter and they seem to be operating correctly with a jack inserted and without. I've poked around and tested various connection and traces for cold solder joints, though I'm guessing that a simple continuity test may not indicate a good solder connection. Also, no obvious buned or shorted items anywhere. Board is very clean.

On the input jack (JS1) check the correctness of the jumper connecting R68/ R69 to gnd to eliminate the noise of the boost channel. During use, it can be mechanically damaged and the boost channel may be be damped.

The fact that the pre-amp tube (V1/ 12AX7) glows does not necessarily mean that it is correct. By checking the voltage at the anodes V1/ 12AX7 (pin1; pin6) you are simultaneously checking the correctness of the HT power supply.

VSMK1 uses Dual Op-Amp - M5201 with switch (IC3; IC5), to switch normal/ boost channels.
The control voltage for channel switching the M5201 receive at pin1 from the transistor TR1 (MPSA13).

Question
Does switching channels to boost does not work in both cases with FSW and manually with panel SW2. In any case, measure the control voltage on pin1 IC3 and IC5 when switching normal/ boost, and whether channel switching follow by LED1 and LED2 diodes.

If the control voltage at pin1 IC3 and IC5 (M5201) changes with the change of SW2, there is a high probability that IC3 or IC5 is faulty.
When replacing IC3 or IC5 installs in 8-Pin DIP socket.

https://www.amplifiedparts.com/products/op-amp-m5201-dual-switch-8-pin-dip
Op-Amp - M5201, Dual, With Switch, 8-Pin DIP

https://drtube.com/schematics/marshall/pc0689.pdf
8100 Valvestate preamp